1. Field of the Invention
The present invention relates to a manufacturing method of a diffractive optical element (“DOE”). The DOE of the present invention is suitable for a semiconductor exposure apparatus, a laser processing apparatus, etc.
2. Description of the Related Art
In a conventional semiconductor exposure apparatus, a DOE is used for an illumination optical system configured to provide an off-axis illumination that improves the resolution (Japanese Patent Laid-Open No. (“JP”) 7-201697). In a laser processing apparatus, the DOE is used for a beam shaper and a beam splitter so as to improve the processing precision and the throughput (JP 2002-221614). The DOE is an optical element made by forming a micro relief structure shape on a transparent substrate. The phase of light that has transmitted the DOE is modulated by a phase distribution that is generated by a difference between a refractive index of a substrate material and a refractive index of air, and converted into a predetermined intensity distribution.
In general, the convexoconcave shape of the DOE is formed by repeating the lithography and etching. A 2n-level (step) structure is manufactured by repeating the lithography and etching n times: The first set of the lithography and the etching provides a two-level structure, and the second set of the lithography and the etching provides a four-stage structure. An optical function having a higher performance can be obtained by increasing the number of stages. However, the increased number of stages would increase the manufacturing cost. In order to restrain the manufacturing cost, the two-level DOE that can provide an intended optical function is demanded.
It is well-known that an iterative Fourier transform algorithm (“IFTA”), which is one of designing methods of the DOE, has a high convergent speed (Frank Wyrowski, “Diffractive optical elements: iterative calculation of quantized, blazed phase structures,” J. Opt. Soc. Am. A, 7, p. 961-969 (1990)).
When the DOE is used for the semiconductor exposure apparatus or the laser processing apparatus, the DOE is often applied to an optical system configured to form a reproduced image on the optical axis. In this case, the reproduced image often has an intensity distribution point-symmetrical with respect to the optical axis, and it is difficult to sufficiently improve the diffraction efficiency of the IFTA-designed, two-level DOE and to sufficiently reduce its noises.